There is a growing wave of public support for renewable energy popularly called “Green Power”. Several well-known companies, according to Power magazine for May 2003, including General Motors, IBM, Dow Chemical and Johnson & Johnson, have announced plans to purchase a portion of their power requirement from “green” sources. Some companies have even announced intentions to replace all of the electricity used in their manufacturing with “green power”. Pillars of fossil energy supply, such as Chevron, British Petroleum (BP) and Shell Oil, have announced their intentions to support environmental causes. In fact, BP is an important supplier of solar energy panels. There is a “Green Power Market Development Group” of the World Resources Institute (WRI), aiming to develop 1,000 Megawatts (MW) of new, cost-competitive “green power” by 2010.
In addition, more than a dozen state legislatures require power marketers to phase in specific and increasing percentages of power from renewable sources. New York has mandated that state agencies must buy 25% of their power from renewable sources by 2013; currently 19.3% of the energy produced in New York is generated from renewable sources (New York Public Service Commission). California has passed legislation requiring that 20% of utilities' electricity in the state be produced from renewable sources by 2017. In fact, one California utility, Pacific Gas and Electricity (PG&E), advertises that more than 30% of its electricity now comes from renewable sources. At least 36 U.S. power retailers now offer a “green power” alternative. Europe also takes renewable energy seriously, targeting 20% of its generation from renewables by 2020.
Conventional renewable energy generally covers origination from solar, wind, hydro-electric, geothermal, biomass and landfill gas. There is some question as to how the demand for renewable energy will be met. Solar and wind are growing, but from a very small base. Hydro-electric and geothermal have limited new sites and face ecological opposition. Landfill gas is limited and also criticized for air pollution. There are currently no other renewable sources which might be tapped to fill the large gap between supply and demand.
Biomass has long been used as a renewable energy source. For example, wood and forestry, as well as agricultural, by-products have been used as fuels for centuries by mechanically firing them in furnaces and boilers with high excess air and low efficiency. The National Renewable Energy Laboratory (NREL) defines biomass as: “organic matter available on a renewable basis. Biomass includes forest and mill residues, agricultural crops and wastes, wood and wood wastes, animal wastes, livestock operation residues, aquatic plants, fast growing trees and plants and municipal and industrial wastes.” According to The Sandia National Laboratory's Combustion Research Facility (CRF), combustion is involved in 85% of the world's energy use. If biomass is to make a meaningful contribution to renewable energy, it will be, directly or indirectly, as a fuel.
Sewage sludge, and the large amounts of biosolids it contains, with their cell-bound water, has not previously been considered an energy source. Due to their large bound water content, biosolids have a negative fuel value and cannot be incinerated unless heated with expensive fuel that must be purchased. Such an incineration of biosolids may be desirable to avoid having to spread them on land, thereby eliminating or at least reducing possible environmental contamination, but at a very substantial cost, namely the additional heat that must come from the fuels to incinerate them.
The production of biosolids in the U.S. is estimated to be between 7.1 and 7.6 million (short) dry tons per year. Ocean dumping has been prohibited since the 1980s. The predominant disposition is spreading the biosolids on agricultural land as a fertilizer. Other dispositions are dumping in landfills and incineration.
In 1998, the production of biosolids in Europe was reported to be 7.2 million dry metric tons, and 25% was disposed to landfills. Production is expected to increase to at least 9.4 million metric tons in 2005, land application accounting for 54%, landfilling decreasing to 19%, and incineration growing to 24%—although incineration is estimated to cost five times as much as landfilling.
In 2001, biosolids production in Japan was reported to be 1.7 million dry metric tons. 40% was composted and the remainder was incinerated or used to produce cement.
After strenuous mechanical dewatering and digestion in sewage treatment plants, the solids concentration in biosolids still only ranges from about 14-30%, and is typically no more than about 20%, which means that every ton of biosolids, treated and dewatered in accordance with the prior art, is accompanied by about four tons of water, the bulk of which is bound in the dead cells. The cost of shipping the inert water limits the distance it can be moved from its source, usually a wastewater treatment plant (WWTP). These factors give biosolids a negative value. As a result, the WWTP must pay to have someone dispose of the biosolids. Such a payment is often called a “tipping fee”.
As the options for biosolids disposal become more challenging and the disposal options are moved farther from the source, disposal costs and transportation costs have become increasingly significant economic burdens. To reduce this burden, industry has focused on volume and weight reduction. The wastewater industry has made extensive efforts to remove the water from the biosolids generated at treatment plants. A typical WWTP may employ centrifuges, belt presses, rotary presses or other processes to physically force the water from the biosolids. A polymer and other chemicals may be added to assist in dewatering. Nevertheless, such mechanical dewatering methods used by WWTPs are inefficient and costly and incapable of appreciably reducing the amount of water bound in the cells of the biosolids.
The U.S. Environmental Protection Agency (EPA) grades biosolids according to regulation “40 CFR Part 503” as Class A and Class B. This regulation concerns primarily the application of biosolids to agricultural land, to which there is vocal and growing environmental opposition. For example, environmentalists condemn the use of biosolids as a fertilizer because of their content of living disease-causing organisms (pathogens and viruses) and heavy metals (such as lead, mercury, cadmium, zinc and nickel), as well as their damage to groundwater quality. In addition, environmentalists raise concerns about “quality of life” issues, such as insects and odors, associated with biosolids. As such, land application of Class B biosolids is banned in a number of counties, and more counties and states are expected to follow. In one case, where 70% of the biosolids were Class B, the banning of land application in adjacent counties nearly doubled the tipping fee from about $125 per dry ton to about $210-$235.
Furthermore, the high cell-bound water content of biosolids makes their incineration difficult for many industries. For example, the cement industry is reputed to be the world's third largest energy user. It requires the equivalent of about 470 pounds of coal to make each ton of cement. To conserve fossil fuel, 15 cement plants in the U.S. burn fuel-quality hazardous waste, and about 35 other plants use scrap tires to supplement fossil fuel. A growing method of disposing of biosolids is to incinerate them in cement kilns. Since their net fuel value is negative, this practice is only viable because of the revenue received by the kiln operator, for example, from the tipping fee, since additional fuel, such as coal, must be fired to eliminate the water bound in biosolids. In addition, in the manufacture of cement, certain elements contained in biosolids, such as chlorine, phosphorus, sodium and potassium, are not desired because they adversely affect the quality of the cement.
In the past, the requirement to dispose of biomass in general was coupled with attempts to extract heat energy from it in order to reduce disposal costs and the environmental burden of landfills. Attempts to extract energy from such materials were limited to combusting low-grade fuels and solid waste. For example, previous processes for deriving fuel from municipal solid waste (MSW) generally focus on adding alkali to assist in the removal of the majority of contained chlorine in the form of PVC found in MSW. In addition, various methods for processing relatively low-grade carbonaceous fuel, such as sub-bituminous and lignite coals, are known to those of ordinary skill in the art. In both scenarios, however, low-grade fuels are used as raw materials.
A number of schemes for the pyrolysis of biosolids have been advanced. However, they all have been forced to contend with the fact that biosolids contain about four times as much water as solid material, even after conventional dewatering at the treatment plant, for example. It is impossible to reach pyrolysis temperatures until all of the water has been vaporized, which requires at least 4000 Btu per pound of solids, which, at best, might be equal to its fuel value, before allowing for capital and operating costs.
As the foregoing demonstrates, the disposal of biosolids has become increasingly expensive and controversial. A need exists in the art for a method to cleanly and economically dispose of biosolids. The current invention provides a method to dispose of biosolids while concurrently producing an economically more viable renewable fuel.
To the extent that biosolids alone cannot meet the growing demand for renewable energy, the biosolids conversion to a useable fuel in according with the present invention can be combined with extracting energy from other sources such as biomass. Thus, the present invention provides a method and system to convert biosolids, alone or with biomass, into a viable renewable fuel in an environmentally benign manner.